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SubjectRe: [PATCH v4] arm64: sysreg: Make mrs_s and msr_s macros work with Clang and LTO
Hi Kees,

On Tue, Apr 23, 2019 at 04:26:22PM -0700, Kees Cook wrote:
> Clang's integrated assembler does not allow assembly macros defined
> in one inline asm block using the .macro directive to be used across
> separate asm blocks. LLVM developers consider this a feature and not a
> bug, recommending code refactoring:
>
> https://bugs.llvm.org/show_bug.cgi?id=19749
>
> As binutils doesn't allow macros to be redefined, this change uses
> UNDEFINE_MRS_S and UNDEFINE_MSR_S to define corresponding macros
> in-place and workaround gcc and clang limitations on redefining macros
> across different assembler blocks.

[...]

> diff --git a/arch/arm64/include/asm/irqflags.h b/arch/arm64/include/asm/irqflags.h
> index 43d8366c1e87..d359f28765cb 100644
> --- a/arch/arm64/include/asm/irqflags.h
> +++ b/arch/arm64/include/asm/irqflags.h
> @@ -43,7 +43,7 @@ static inline void arch_local_irq_enable(void)
> asm volatile(ALTERNATIVE(
> "msr daifclr, #2 // arch_local_irq_enable\n"
> "nop",
> - "msr_s " __stringify(SYS_ICC_PMR_EL1) ",%0\n"
> + __msr_s(SYS_ICC_PMR_EL1)

Seeing the __stringify() go is nice!

Our assembly happens to use argument 0 by coincidence rather than by deliberate
design, so please have the macro take the template, e.g.

__msr_s(SYS_ICC_PMR_EL1, "%x0")

... that ways it's obvious as to which asm parameter is used, and this won't
complicate refactoring of the assembly (which may shuffle or rename the
parameters).

Likewise for __mrs_s(), e.g.

__mrs_s("%x[va]", SYS_WHATEVER);

[...]

> +#define __mrs_s(r) \
> + DEFINE_MRS_S \
> +" mrs_s %0, " __stringify(r) "\n" \
> + UNDEFINE_MRS_S
> +
> +#define __msr_s(r) \
> + DEFINE_MSR_S \
> +" msr_s " __stringify(r) ", %0\n" \
> + UNDEFINE_MSR_S

These can be:

#define __mrs_s(v, r) \
DEFINE_MRS_S \
" mrs_s " v ", " __stringify(r) "\n" \
UNDEFINE_MRS_S


#define __msr_s(r, v) \
DEFINE_MSR_S \
" msr_s " __stringify(r) ", " v "\n" \
UNDEFINE_MSR_S


> +
> +/* For use with "rZ" constraints. */
> +#define __msr_s_x0(r) \
> + DEFINE_MSR_S \
> +" msr_s " __stringify(r) ", %x0\n" \
> + UNDEFINE_MSR_S

I don't think we need this. If we pass the template in, this is solved by the
caller, and we could consistently use the x form regardless.

Otherwise, I think this is as clean as we can make this short of bumping our
minimum binutils version and using the S<Op0>_<...> names.

Thanks,
Mark.

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